Re: SSD & mechanical disc in RAID 1

[Bill Davidsen <davidsen@tmr.com>]

Goswin von Brederlow wrote:
> Keld Jørn Simonsen <keld@keldix.com> writes:
>
>   
>> On Sat, Jan 09, 2010 at 09:53:11AM -0800, Wil Reichert wrote:
>>     
>>> Has anyone ever tried putting an SSD and a mechanical disc in RAID 1
>>> using write-mostly?  The goal would be to extol the speed & latency
>>> virtues of an SSD while retaining the integrity of the traditional
>>> storage.  Would this setup even work as I expect it to?
>>>       
>> I think you are better served by trying out RAID10 in the far or offset
>> layouts. RAID1 would per se not gain from the low latency times of 
>> SSD. RAID1 should function the same with hard disk as with SSD, 
>> and with hard disk it would not make sense to multiplex reading
>> of a file, as skipping and reading a block essentially takes the same
>> time.
>>
>> Best regards
>> Keld
>>     
>
> With raid10 in the far or offset layout every (large) read and write
> would always access both the SSD and rotating disk. That would make
> every operation wait for the rotating disk to seek.
>
>
> I think his initial idea of raid1 is verry good. With write-mostly all
> reads should come from the fast SSD. I would also add write-behind.
> That way writes will not wait for the rotating disk and get the full
> SSD speed as well.
>   

I had an SSD waiting to be deployed, and I tried putting a journal on it 
and using a mount with data=journal. Supposedly when the data is written 
to the journal the write will be "complete" and effective write speed 
for many small random writes will be higher. I don't have the results 
handy, but they were not so great that I went out and got another SDD.

-- 
Bill Davidsen <davidsen@tmr.com>
  "We can't solve today's problems by using the same thinking we
   used in creating them." - Einstein


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